Historically, compressing refrigerant gas in the compressor size range below 200 kW has been carried out by motor driven positive displacement machines—e.g., piston, vane, screw. Centrifugal compressors currently used are very large, as they must rotate at moderate rotational speeds with high compressor rotor tip speeds in order to be efficient. While such centrifugal compressor can offer efficient operation, they should not be operated at high rotational speeds. High rotation speeds, however, are desirable because the compressors, and therefore the technology with which the compressors are integrated, can be made smaller while still maintaining the same compressed gas flows and pressures and overall efficiency of operation. Requirements for running at high speeds include properly designed machines running at 20,000 to 75,000 rpm.
High-speed rotating machines supported on foil air bearings have made significant progress in the last thirty years. Reliability of high-speed rotating machines with foil bearings has shown a tenfold improvement compared to designs using rolling element bearings.
The use of foil air bearings in centrifugal compressors for refrigeration applications has several advantages:
Oil Free Operation: Typical gas compressors use oil as a lubricant for the compressor bearing. With foil air bearings, there is no miscibility problem between refrigerant and oil requiring oil management, no chemical reaction between oil and refrigerant, no degradation of heat transfer surfaces in the evaporator coils, and no oil running through the components of the compressor.
Higher Reliability: Foil gas bearing machines are more reliable because fewer parts are necessary and no lubrication feeding system is required. In operation, the gas film between the bearing and the motor driven shaft protects the bearing foil from wear. The bearing surface and the shaft are only in contact at start and stop of the machine. In these brief moments, special coating protects the foil against wear.
No Scheduled Maintenance: Since a foil gas bearing machine does not require oil lubricant there is no need for monitoring and replacing the oil.
Environmental and System Durability: Foil gas bearings can handle severe environmental conditions such as shock and vibration loading. Any liquid from the system can also be easily handled without detrimental effect on the bearings.
High Speed Operation: Compressor rotors have better aerodynamic efficiency at higher speeds. Foil gas bearings allow such machines to operate at higher rotational speeds without any limitations as opposed to ball bearings. Due to the hydrodynamic action, foil gas bearings also have higher load capacity as speed increases.
Low and High Temperature Capabilities: Oil lubricants cannot operate at very high temperatures without breaking down. At low temperatures oil lubricants become too viscous to function effectively. Foil air bearings, by comparison, operate efficiently both at severely high temperatures and at cryogenic temperatures.
Incorporation of foil gas bearings into motor driven rotating machines, such as compressors, has been difficult because of additional technologies that are required for efficient operation. For example, the foil bearings must have higher spring rate to compensate for negative spring rate for the motor rotor. Further, sufficient cooling flow between rotor shaft and motor stator is needed to remove heat generated by the motor. An effective cooling scheme is also required for the motor stator. Further, a high-frequency controller is required to drive the motor and maintain the desired operational speeds.